The present invention relates to optics for use in a raster output scanner (ROS) for creating electrostatic latent images from electronic data. More specifically, the invention relates to a low-cost, multi-aperture optical element for a compact, low-error ROS system.
Electrophotographic printers wherein a laser scan line is projected onto a photoconductive surface are well known. In the case of laser printers, facsimile machines, and the like, it is common to employ a raster output scanner (ROS) as a source of signals to be imaged on photographic film or an electrostatically charged photoreceptor (a photosensitive plate, belt, or drum) for purposes of printing. The ROS provides a laser beam which switches on and off according to electronic image data associated with the desired image to be printed as the beam moves, or scans, across the charged photoreceptor. Commonly, the surface of the photoreceptor is selectively imagewise discharged by the laser beam in locations to be printed white, to form the desired image on the photoreceptor. Modulation of the scanned beam creating the desired latent image on the photoreceptor is typically implemented by digitally controlling the output of a high speed laser diode or a modulator associated with a continuous laser source. A common technique for deflecting the modulated laser beam to form a scanline across the photoreceptor surface uses a rotating optical polygon with reflecting surfaces; the laser beam from the ROS is reflected by the facets of the polygon, creating a scanning motion of the beam, which is optically focused to form a sharp scan line across the photoreceptor. A closely spaced regular array of scan lines on a photoreceptor together form a raster of the desired latent image. Once a latent image is formed on the photoreceptor, the latent image is subsequently developed with toner, and the developed image is transferred to a copy sheet, as in the well-known process of electrophotography.
FIG. 8 shows the basic configuration of a scanning system used, for example, in an electrophotographic printer or facsimile machine. A laser source 10 produces a collimated laser beam, also referred to as a "writing beam," 12 which is reflected by the facets of a rotating polygon 14. Each facet of the polygon 14 in turn deflects the writing beam 12 to create an illuminated spot 16 on the pre-charged surface of photoreceptor 18, which in this case is a moving belt. Laser source 10 also includes means for modulating the beam 12 according to image data entered therein. The localized light flux in spot 16 incident at a particular location on the surface of photoreceptor 18, corresponding to a picture element (pixel) in the desired image, discharges the surface for pixels of the desired image which are to be printed white. In locations having pixels which are to be printed black, writing beam 12 is momentarily interrupted through the action of the modulator within source 10, so that the pixel at that location on the surface of photoreceptor 18 will not be discharged. It is to be understood that gray levels are typically imaged in like manner by utilizing exposure levels intermediate between the "on" and "off" levels. Thus, digital data input into laser source 10 is rendered line by line as an electrostatic latent image on the photoreceptor 18.
The rotational motion of polygon 14 results in a spot 16 moving across photoreceptor 18 to form a scan line 20 of selectively discharged areas on photoreceptor 18; polygon 14 thus acts as a "scanner," which serves as a "point source" for the writing beam 12, as those terms are used in the claims herein. At the same time, the surface of photoreceptor 18 is slowly translated at a constant velocity so that the periodic scanning of spot 16 across the moving photoreceptor 18 creates an evenly spaced closely spaced array of scan lines 20, called a raster 22, on the photoreceptor 18, forming the desired continuous image to be printed. One skilled in the art will appreciate that such a configuration has traditionally further included any number of lenses, mirrors and translational mechanisms to accommodate a specific design.
In recent years, it has become accepted in the office-equipment industry that the upcoming standard resolution for printed documents will be as high as 600 spots per inch (SPI). As the desired resolution from raster output scanners becomes ever greater, certain practical constraints are pushed to their limits. For example, at a resolution of 600 SPI, the spot-to-spot placement of pixel data in discharged areas on a latent image is 42.3 .mu.m. This means that the diameter of a writing beam 12 for selectively charging or discharging areas of a photoreceptor 18 cannot be appreciably larger than this amount, and is preferably of this size or smaller. Further, this very narrow beam must be deflected to generate straight scan paths 20 to form a coherent raster 22; thus, it is essential to avoid misplacements of the scan paths 20 which could be caused by small errors in the facet angles of the polygon 14, or the inevitable wobble which occurs as a result of motor bearing runout, dynamic strains, and vibration when polygon 14 rotates at high speed. The combination of ever finer resolution (the optimum exposing spot being smaller in area and the raster spacing being closer together will take a larger number of scans, each with more data points per inch, to fully process a page with a desired image) and ever faster necessary speeds and precision in the raster geometry generated by polygon 14, create a distinct need for novel solutions to allow high-resolution raster output scanners to be created without undue expense.
U.S. Pat. No. 4,274,703 discloses an optical system for a raster output scanner wherein each facet of the polygon subtends a sufficient angle about the axis of rotation of the polygon to ensure that an input beam remains fully seated on a single facet while the reflected beam is being scanned through a desired scan angle. There is provided in the path of the writing beam a spherical focusing lens and at least one cylindrical sagittal correction lens. The curvature of the spherical lens is selected to compensate for the tendency of the scanning spot velocity to vary as a function of the field position.
U.S. Pat No. 4,578,615 discloses a printing device in which a light source for a photoreceptor is in the form of a planar grid of addressable phosphor elements. The elements are arranged in the grid in different locations along the process direction of the photoreceptor, but each element is disposed in a unique location across the photoreceptor, so that a full line of pixels may be formed on the photoreceptor as the photoreceptor moves in the process direction. A fly's-eye lens is employed as a light-coupling means with the printing device.